The sizes of solid polymer fuel cells, having polymer electrolyte membranes, can be easily reduced. The solid polymer fuel cells are thus expected to be applied to mobile vehicles such as electric cars and power sources for small cogeneration systems. However, the solid polymer fuel cells operate at relatively low temperatures. Further, it is difficult to effectively utilize waste heat from them for auxiliary power or the like. Accordingly, to be put to practical use, the solid polymer fuel cells need to offer a high generation efficiency and a high power density under operating conditions including a high anode reaction gas (pure water or the like) utilization rate and a high cathode reaction gas (air or the like) utilization rate.
An electrode reaction in a catalyst layer in each of the anode and cathode of the solid polymer fuel cell occurs at a three phase interface (hereinafter referred to as a reaction site) where reaction gases, a catalyst, and a fluorine containing ion exchange resin are simultaneously present. Thus, the reaction in each electrode occurs only at the three phase interface, where gas (hydrogen or oxygen) corresponding to an active substance, protons (H+), and electrons (e−) can be simultaneously transferred to one another.
An example of an electrode having this function is a solid polymer electrode-catalyst composite electrode containing a solid polymer electrolyte, carbon particles, and a catalytic substance. For example, in this electrode, the carbon particles carrying the catalytic substance are mixed with the solid polymer electrolyte so that the carbon particles, catalytic substance, and solid polymer electrolyte are three-dimensionally distributed. Further, a plurality of pores are formed inside the electrode, which is thus porous. The carbon, a carrier of the catalyst, forms an electron conducting channel. The solid electrolyte forms a proton conducting channel. The pores form a supply and discharge channel for oxygen, hydrogen or water. These three channels spread three-dimensionally in the electrode to form countless three phase interfaces, where the gas, protons (H+), and electrons (e−) can be simultaneously transferred to one another. This provides a field for electrode reactions.
Thus, for the conventional solid polymer fuel cells, a catalyst such as a metal catalyst or a metal carrying catalyst (for example, metal carrying carbon comprising a carbon black carrier with a large specific surface area and a metal catalyst such as platinum carried by the carbon, black carrier) is coated with the same fluorine containing ion exchange resin as or a fluorine containing ion change resin different from that contained in the polymer electrolyte membrane. The catalyst coated with the fluorine containing ion exchange resin is then used as a component of the catalyst layer to perform what is called an operation of making the reaction sites in the catalyst layer three-dimensional. This increases the number of reaction sites and improves the utilization efficiency of expensive noble metal such as platinum, corresponding to the catalytic metal.
Putting fuel cell cars to practical use requires a drastic reduction in costs. However, with the conventional fuel cell catalysts, a reduction in the amount of noble metal in one of the anode and cathode may disadvantageously sharply reduce resultant power owing to the very high activity of the noble metal.
Thus, to reduce the amount of catalyst, JP Patent Publication (Kokai) No. 8-148151 A (1996) discloses the invention of a fuel cell electrode comprising a catalyst layer formed on the gas diffusion layer and containing catalytic particles carrying an active metal, wherein the catalyst layer comprises multiple layers of catalytic particles of different carried active metal amounts.
Thus, the conventional techniques for reducing the noble metal amount focus on the improvement of the electrode structure and few of them take note of the physical properties of the electrode catalyst itself.